How to add custom objects to the "add object" menu?
There is an addon called creaprim which allows you to do this.

Install and enable the addon (direct download). Some people have had issues using this addon in the 2.70, you can find a patched version here.

Select the object which you wish to add to the Add menu and press Create Primitive in Tools shelf &gt; Tool &gt; History &gt; CreaPrim:


You can now add it from the Add menu: